The default is some kind of Jacobi plus Chebyshev, for a certain class of problems, it is quite good.
On Jan 10, 2023, at 3:31 PM, Mark Lohry <[email protected]> wrote:
So what are people using for GAMG configs on GPU? I was hoping petsc today would be performance competitive with AMGx but it sounds like that's not the case?
On Tue, Jan 10, 2023 at 3:03 PM Jed Brown <[email protected] <mailto:[email protected]>> wrote:
Mark Lohry <[email protected] <mailto:[email protected]>> writes:
I definitely need multigrid. I was under the impression that GAMG was relatively cuda-complete, is that not the case? What functionality works fully on GPU and what doesn't, without any host transfers (aside from what's needed for MPI)?
If I use -ksp-pc_type gamg -mg_levels_pc_type pbjacobi -mg_levels_ksp_type richardson is that fully on device, but -mg_levels_pc_type ilu or -mg_levels_pc_type sor require transfers?
You can do `-mg_levels_pc_type ilu`, but it'll be extremely slow (like 20x slower than an operator apply). One can use Krylov smoothers, though that's more synchronization. Automatic construction of operator-dependent multistage smoothers for linear multigrid (because Chebyshev only works for problems that have eigenvalues near the real axis) is something I've wanted to develop for at least a decade, but time is always short. I might put some effort into p-MG with such smoothers this year as we add DDES to our scale-resolving compressible solver.